The present invention relates to a channel selection apparatus, a receiver and a channel selection method, respectively for receiving and reproducing digital TV broadcast data by sweeping a predetermined frequency range. More particularly, the invention relates to a channel selection apparatus, a receiver and a channel selection method, respectively for receiving digital TV broadcast data, capable of starting providing services by presetting all receivable (tunable) stations in a short time.
A channel selection apparatus is known which sweeps a predetermined frequency range in a broadcast band and tunes in a desired channel to receive digital TV broadcast. A channel selection apparatus of this type establishes synchronization by sweeping a predetermined frequency range and derives a MPEG2 (Moving Picture Expert Group Phase 2) transport stream signal from an IF (Intermediate Frequency) signal in order to provide services.
Such a channel selection apparatus for receiving digital TV broadcast presets receivable channels in the following manner. The frequency of a local oscillation signal is sequentially changed in the unit of a predetermined frequency range capable of AFC (Automatic Frequency Control). Each time the oscillation frequency is changed, a received signal is demodulated to monitor a signal reception state of various circuits constituting a demodulation circuit. Namely, the channel selection apparatus monitors a carrier lock state by a carrier reproduction circuit, a clock signal lock state by a clock reproduction circuit, an error rate in a Viterbi decoder and a Reed Solomon decoder, and a detection state of a synchronization pattern (synchronization byte) of an MPEG2 transport stream signal by a de-scrambler. The channel selection apparatus monitors a lock state, error rate and the like of such signals to judge whether data is being reproduced correctly. If it is decided that the data is not reproduced correctly, a channel selection is performed in the next predetermined frequency range. If it is decided that the data is being produced correctly, the channel selection apparatus stores the tuned frequency to preset it and make it easy to tune in the channel at the next channel selection.
With the above-described conventional techniques, each time the frequency of a local oscillation signal is changed, received data is demodulated and the state of various signals in the demodulation circuit is monitored to judge whether data is being reproduced correctly. This method is therefore associated with the problem that a time taken to perform a channel selection is longer than that for an analog channel selection. Since a defined band, particularly of a satellite broadcast band, is broad (950 MHz to 2150 MHz), it takes a very long time to sweep the whole band in which digital broadcast services are transmitted, to select channels and to preset frequency data and the like.
The present invention has been made under the above-described circumstance and provides a channel selection apparatus, a receiver and a channel selection method, respectively for receiving digital TV broadcast data, capable of starting providing services by presetting all receivable channels in a short time.
In order to achieve the above object of the invention, a channel selection apparatus, according to a first aspect of the invention, for digital TV broadcast use for receiving a first intermediate frequency (IF) signal and for automatically sweeping a predetermined frequency range for tuning, said apparatus comprises:
oscillation signal creation means for creating a local oscillation signal to be used for changing the frequency of the intermediate frequency signal, the oscillation signal creation means being made of a PLL synthesizer and the like;
frequency conversion means for converting the frequency of the intermediate frequency signal by using the local oscillation signal created by the oscillation signal creation means, the frequency conversion means being made of a mixer and the like;
data demodulation means for demodulating data in the intermediate frequency signal whose frequency was converted by the frequency conversion means, the data modulation means being made of a quadrature detection unit, an A/D converter, an interpolation unit, a roll-off filter and the like; data reproduction means for reproducing service data for providing a service from the data demodulated by the data demodulation means, the data reproduction means being made of a Viterbi decoder, a convolution de-interleaver, a Reed Solomon decoder, a de-scrambler and the like;
signal level detection means for detecting a signal level of the data demodulated by the data demodulation means; and
operation switching means for deciding whether the signal level detected by said signal level detection means is larger than a reference value, said operation switching means being operable to change the frequency of the local oscillation signal by an amount corresponding to said predetermined frequency range without subsequently operating said reproduction means so as operation in a new frequency range, if it is decided that the signal level detected by said signal level means is equal to or smaller than the reference level, and data reproduction means for reproducing service data for providing a service from the data demodulated by said data demodulation means.
According to the invention, the oscillation signal creation means generates a local oscillation signal to be used for changing the frequency of the received intermediate frequency signal, and the data demodulation means demodulates data in the intermediate frequency signal whose frequency was converted by the frequency conversion means. The data reproduction means reproduces service data for providing a service from the data demodulated by the data demodulation means. The signal level detection means detects a signal level of the data demodulated by the data demodulation means, and the operation switching means decides whether the signal level detected by the signal level detection means is larger than a reference value. The signal level detection means changes the frequency of the local oscillation signal if it is decided that the signal level detected by the signal level detection means is equal to or smaller than the reference level, and performs a channel selection in a new frequency range. Accordingly, if the signal level of the data demodulated by the data demodulation means is equal to or smaller than the reference value, the next frequency range is immediately swept for a channel selection without reproducing the data for providing a service. It is therefore possible to perform a channel selection in a short time.
If it is decided that the signal level detected by the signal level detection means is larger than the reference value, the operation switching makes the data reproduction means reproduce service data for the first time.
It is preferable that:
the data reproduction means comprises signal amplifier means for amplifying the intermediate frequency signal whose frequency was converted by the frequency conversion means, the signal amplifier means being made of an AGC amplifier and the like; and
the signal level detection means comprises means for feeding back a voltage to the signal amplifier means, the voltage defining a gain of the signal amplifier means to make constant a power of an input signal to the data modulation means.
Accordingly, the signal level detection means can detect the signal level only when data is demodulated. By detecting the signal level quickly and switching between operations, time taken to select a channel can be shortened.
A channel selection method, according to a second aspect of the invention, for digital TV broadcast use, for receiving a first intermediate frequency (IF) signal and automatically sweeping a predetermined frequency range for tuning said method comprises:
an oscillation signal creation step of creating a local oscillation signal to be used for changing the frequency of the intermediate frequency signal;
a frequency conversion step of changing the frequency of the intermediate frequency signal by using the local oscillation signal created by the oscillation signal creation step;
a data demodulation step of demodulating data in the intermediate frequency whose frequency was changed by the frequency conversion step;
a signal level detection step of detecting a signal level of the data demodulated by the data demodulation step;
a signal level deciding step of deciding whether the signal level detected by the signal level detection step is larger than a reference value;
operation switching step of changing the frequency of the local oscillation signal by an amount corresponding to said predetermined frequency range to return said frequency conversion step, if it is decided that the signal level detected in said signal level is equal to or smaller than the reference level, and;
a data reproduction step of reproducing service data for providing a service from the data demodulated by the data demodulation step if the signal level deciding step decides that the signal level is larger than the reference level.
According to the invention, the oscillation signal creation step generates a local oscillation signal to be used for changing the frequency of a received intermediate frequency signal. The frequency conversion step changes the frequency of the intermediate frequency signal by using the local oscillation signal created by the oscillation signal creation step. The date demodulation step demodulates data in the intermediate frequency whose frequency was changed by the frequency conversion step. The signal level detection step detects a signal level of the data demodulated by the data demodulation step. The signal level deciding step decides whether the signal level detected by the signal level detection step is larger than a reference value. The operation switching means operate to change the frequency of the level oscillation without moving to the subsequent reproduction step, so as to sweep a new frequency range if it is decided that the signal level detected in said signal level is equal to or smaller than the reference level. The data reproduction step reproduces service data for providing a service from the data demodulated by the data demodulation step if the signal level deciding step decides that the signal level is larger than the reference level. Accordingly, if the signal level of the data demodulated by the data demodulation step is equal to or smaller than the reference value, the next frequency range is immediately swept for a channel selection without reproducing service data. Therefore, it becomes possible to remarkably shorter the time taken to sweep the whole band.